Bipolar transistors are known in the art. Wide bandgap bipolar transistors are known in the art as Heterojunction Bipolar Transistors (HBTs). The bandgap of an HBT's emitter region is wider than the bandgap of its base region. This bandgap difference in an npn HBT creates a barrier to the hole injection from the base to the emitter. Consequently, HBTs have a high emitter injection efficiency, .alpha., and a correspondingly high static current gain, .beta.. Thus, HBTs have a high current drive capability, a low sensitivity to output loading, and a turn-on voltage that is nearly independent of junction current. HBT design is, primarily, a balance of 3 design parameters: the static current gain .beta.; base resistance R.sub.b ; and the emitter-base junction capacitance, C.sub.eb.
HBT's perform better than other types of bipolar transistors in high-frequency, microwave applications. At high frequencies, other types of Bipolar transistors are lossy and inefficient. To compensate for that inefficiency at high frequency, in other types of bipolar transistors the ratio of the doping concentrations in the emitter and the base is made large, and the emitter width is made small. Because they have a higher current gain and are not sensitive to switching speed, HBTs may remain small, providing an advantage over other types of Bipolar transistors.
However, prior art HBTs also have problems. For example, Al.sub.z Ga.sub.1-z As/GaAs HBT 10 (where z is the mole fraction of aluminum in AlGaAs) have a nonplanar structure as shown in FIG. 1, requiring a complex fabrication process. The HBT 10 has an N-type emitter 12, p.sup.+ base 14, and N-type collector 16 on an N.sup.+ subcollector 18. The subcollector 18 is on a semi-insulating GaAs substrate 20. The emitter 12 has an N.sup.+ cap 22 and SiO.sub.2 side walls 24. The emitter 12, base 14 and collector 16 have metal or resistive contacts 26, 28, and 30, respectively. Because of its nonplanar structure, the regions between the HBT's emitter 12 and extrinsic base 14 are exposed, acting as electron-hole recombination sources.
Other prior art HBT structures have equally troublesome electron-hole recombination sources. For example, there is a high lattice mismatch between Si and Ge in the SiGe base for Si/Ge.sub.x Si.sub.1-x /Si HBTs (x being the Ge mole fraction in SiGe). This mismatch causes strain at Si/SiGe interfaces, both at the emitter-base and the base-collector junctions. This strain is a source of electron-hole recombination in each junction's space charge region. Consequently, because of that recombination, these prior art HBTs had reduced current density.